The pressure of any permanent tailwater above the plane considered
will similarly give rise to the corresponding resultant forces PwhandPwv
operative above the toe, as illustrated on the figure, andPwh (^) wz^22 /2 (3.2a)
with
Pwv (^) w(areaA 2 ). (3.3a)
Unit weight (^) w10 kN m^3 is frequently adopted for convenience in
analysis. In a small number of instances, e.g. for flood diversion dams,
exceptional suspended sediment concentrations following extreme flood
events have been used to justify a further marginal increase in (^) w.
SELF-WEIGHT LOAD
Structure self-weight is accounted for in terms of its resultant, Pm, which is
considered to act through the centroid of the cross-sectional area Apof the
dam profile:
Pm (^) cAp(kN m^1 ). (3.4)
(^) cis the unit weight of concrete, assumed as 23.5 kN m^3 in the absence of
specific data from laboratory trials or from core samples. Where crest
gates and other ancillary structures or equipment of significant weight are
present they must also be accounted for in determining Pmand the
position of its line of action.
SEEPAGE AND UPLIFT LOAD
Interstitial water pressures, uw, develop within a concrete dam and its
foundation as a result of preferential water penetration along discontinu-
ities, e.g. joint planes, cracks and fine fissures, and also by seepage within
the pore structure of rock and concrete. The pressures are directly analo-
gous to porewater pressures in soil mechanics (Chapter 2), and symbol uw
is employed in this chapter for consistency.
The theoretical long-term equilibrium pressure distribution within an
‘ideal’, i.e. homogeneous and intact, dam and foundation can be estab-
lished from a flownet such as in the example sketched in Fig. 3.2(a). A
curvilinear distribution of pressure exists across a horizontal plane, as indi-
cated in Fig. 3.2(b). In practice, pressure distributions are governed by the
nature and frequency of the random discontinuities present, e.g. joints,
microfissures etc., and are thus locally indeterminate. It is therefore